1. Field of the Invention
The present invention relates to a heat activation method for a thermosensitive adhesive label comprising a support, and a thermosensitive adhesive layer which is provided on the support without a liner (i.e., a disposable backing sheet) and is not adhesive at room temperature, but can be made adhesive with the application of heat thereto.
The present invention also relates to an apparatus for heat-activating the above-mentioned thermosensitive adhesive layer of the thermosensitive adhesive label and a label printer capable of printing images on the thermosensitive adhesive label and heat-activating the thermosensitive adhesive layer thereof.
2. Discussion of Background
Recently, a recording label, in particular, a thermosensitive recording label has been used in a wide variety of fields, for example, in the system of point of sales (POS). In most of the above-mentioned conventional thermosensitive recording labels, a pressure-sensitive adhesive layer is generally provided on the back side of a thermosensitive recording layer, so that the label is stored in such a fashion that a liner (i.e., disposable backing sheet) is attached to the pressure-sensitive adhesive layer.
Such a thermosensitive recording label is useful, but it has some shortcomings. For instance, the liner must be discarded after released from the adhesive layer. Therefore, consideration must be given to the problem of waste disposal from the ecological viewpoint. In addition, the manufacturing cost is increased because of not only the cost of the linear itself, but also expenses involved by the treatment of the liner.
To solve the above-mentioned problems, there are proposed recording labels without a liner. For instance, as disclosed in Japanese Laid-Open Utility Model Applications 59-43979 and 59-46265 and Japanese Laid-Open Patent Application 60-54842, it is proposed to employ an adhesive layer comprising a pressure-sensitive adhesive in micro-capsule form, and to provide a releasing agent layer on the surface of the recording label, opposite to the pressure-sensitive adhesive layer side so that the recording label may be stored in the form of a roll. According to the above-mentioned conventional proposals, however, sufficient adhesion is not generated in the pressure-sensitive adhesive prepared in the form of micro-capsules, and printing cannot be carried out on the surface of the label when the releasing agent layer is provided thereon.
Furthermore, there is proposed a thermosensitive recording label comprising a thermosensitive adhesive layer without a liner, as disclosed in Japanese Laid-Open Patent Application 63-303387 and Japanese Utility Model Publication 5-11573. When a recording label comprises a thermosensitive adhesive layer, heat-activation treatment of the thermosensitive adhesive layer becomes necessary. With respect to the above-mentioned heat activation treatment, the following methods are conventionally proposed: the application of hot air or infrared rays to the thermosensitive adhesive layer (Japanese Utility Model Publication 5-11573), the use of an electrical heater or induction coil (Japanese Laid-Open Patent Application 5-127598), the application of microwave to the thermosensitive adhesive layer (Japanese Laid-Open Patent Application 6-8977), the application of xenon flash to the thermosensitive adhesive layer (Japanese Laid-Open Patent Application 7-121108), and the application of halogen lamp to the thermosensitive adhesive layer (Japanese Laid-Open Patent Application 7-164750). Those heat activation methods have the drawbacks that it is difficult to efficiently transmit the thermal energy to the thermosensitive adhesive layer of the label. Therefore, the energy cost is increased because the thermal energy cannot be efficiently utilized. Further, the heat activation apparatus is easily overheated in the course of heat activation. In light of the energy cost and safety, it is proposed to enclose the heater with a heat insulating material. In this case, however, the heat activation apparatus cannot be made compact.
On the other hand, there is also known a heat activation method of the thermosensitive adhesive layer by bringing the thermosensitive adhesive layer into contact with a heating medium or a heat-transmitting medium heated by the heating medium. For example, a heat-application belt heated by a heater is brought into contact with the thermosensitive adhesive layer in Japanese Laid-Open Patent Application 57-37534; and a heated drum and a heated roller serving as the heating media are respectively used in Japanese Laid-Open Patent Applications 60-45132 and 6-263128.
When the thermosensitive adhesive layer is heat-activated by directly bringing it into contact with the above-mentioned heating medium such as a heated drum or roller, and heat-transmitting medium such as a heat-application belt, it is necessary that those heating media be preheated to carry out the heat activation in a heated condition in the interest of speedy heat activation operation. This involves the problem of safety.
In addition, the heat-activated thermosensitive adhesive tends to adhere to the surface of the heating medium or heat-transmitting medium after the completion of the heat activation. Because of such adhesion of the thermosensitive adhesive layer to the heating medium, the recording label itself will unfavorably wind round the heating medium.
Furthermore, when the above-mentioned thermosensitive adhesive label further comprises a thermosensitive coloring layer, the heat resistance of the thermosensitive coloring layer must be improved because the thermosensitive coloring layer easily causes the color development by the application of thermal energy in the course of heat activation. The increase of the heat resistance of the thermosensitive coloring layer is disadvantageous from the viewpoint of thermal sensitivity of the coloring layer.
There is disclosed a heat activation method in Japanese Laid-Open Patent Application 7-258613. According to this method, a heating medium is brought into pressure contact with a support of a recording label to heat-activate a thermosensitive adhesive layer provided on the support. This method has the advantages that it is possible to prevent the thermosensitive adhesive layer from being transferred to the heating medium and the recording label from sticking to the heating medium. However, a lot of thermal energy is wasted, and the heat activation of the thermosensitive adhesive layer cannot be smoothly completed. Therefore, the operating efficiency is lowered when the recording label is attached to a label-receiving member after heat-activating the thermosensitive adhesive layer of the recording label.